If we use protected content, we're at the mercy of a single Cease and Desist letter from WotC, and WotC is legally obligated to send them (if you don't protect a copyright you lose the copyright - it's why Kleenex stopped being a brand name, and arguably why the OGL exists - DnD was about to go non-copyright due to not enforcing their copyright (TSR...).
As an IP law lecturer, I have to say *ouch*.
I think if you are going to make and market this product, you will need to read up on trade marks and copyrights, and/or talk to someone knowledgeable. The author of OSRIC took legal advice before proceeding with what at the time was a radical innovation.
OK, sounds good. Just a note that it is Trade Marks that have to be enforced or risk being lost, not Copyright.